The present invention relates generally to power amplifier circuits, and more particularly, to an envelope elimination and restoration type power amplifier for amplifying a variable envelope input signal using bandlimited signal component estimates.
Several types of digital modulation schemes used in the transmission of radio signals, like Quadrature Amplitude Modulation (QAM) and Quadrature Phase Shift Keying (QPSK) require the use of variable envelope radio frequency signals. The generation and amplification, especially power amplification, of variable envelope radio frequency signals tends to be less efficient and consumes a large percentage of the power in a two way radio. In order to accommodate the amplification of signals having a variable envelope, a power amplifier, which maintains linearity over a wider operating range needs to be used. Most power amplifiers can be tuned to be more efficient at a particular point along the required operating range, often at the expense of efficiency at other points along the operating range.
However some amplifiers can be adapted to operate more efficiently over wider operating ranges than other amplifiers. One such amplifier is an Envelope Elimination and Restoration (EER) amplifier. An EER amplifier generates a signal component representative of a input signal""s envelope and couples the signal to the supply input of a power amplifier. The EER amplifier will also generate a signal component representative of the input signal""s phase and couples the signal to the signal input of the power amplifier. As a result, the component of the input signal representative of the signal""s phase will have it""s amplitude adjusted corresponding to the value of the component of the input signal representative of the signal""s envelope, which is coupled to the supply input of the power amplifier.
However, EER amplifiers have seen limited use, because EER amplifiers have significant concerns regarding linearity and bandwidth. While a bandlimited RF signal will generally have baseband signal components in the cartesian domain that are bandlimited, generally the amplitude and phase component of the bandlimited RF signal will not be bandlimited. As a result in order to accommodate non-bandlimited signal components, the bandwidth requirements of the power amplifier system increases greatly. Additionally linearity problems may arise from amplitude modulation to phase modulation distortion due to nulls in the power supply.
The present inventors have recognized that if the EER amplifier can be accommodated with bandlimited approximations of the amplitude and phase/frequency, the substantial bandwidth increases to the system are no longer necessary, and steps can be taken to adjust the approximation algorithm, to incorporate a favorable predistortion, which accommodates the linearity concerns.
The present invention provides a power amplifier circuit for receiving a variable envelope input signal and for producing an amplified output signal. The power amplifier circuit includes an envelope approximation circuit, an envelope amplifier circuit, a phasor approximation circuit, a quadrature modulation circuit, and a power amplifier. The envelope approximation circuit receives the variable envelope input signal and produces a bandlimited estimated envelope signal, corresponding to the amplitude of the variable envelope input signal. The bandlimited estimated envelope signal is then amplified by an envelope amplifier circuit. The amplified envelope signal is then coupled to the supply input of the power amplifer.
The phasor approximation circuit receives the variable envelope input signal and produces a bandlimited estimated phasor signal. The quadrature modulation circuit receives the estimated phase signal and produces a modulated phase signal. The modulated phase signal is then coupled to the signal input of the power amplifier. The power amplifier then produces an amplified output signal.
In at least one embodiment the estimated amplitude signal and the estimated phase signal are polynomial approximations of the corresponding amplitude and phase signals.
In a further embodiment a bandlimited approximation of frequency is determined as opposed to a bandlimited approximation of phase, and a frequency modulation circuit receives the estimated frequency signal and produces a modulated frequency signal, which is then coupled to the signal input of the power amplifier.
The present invention further provides a method of amplifying a variable envelope input signal. The method includes receiving a variable envelope input signal having at least a pair of baseband signal components. A bandlimited approximation of the amplitude of the variable envelope input signal is then computed from the baseband signal components of the variable envelope input signal. The approximation of the amplitude is then amplified, and the amplified approximation of the amplitude is coupled to the supply input of a power amplifier. The method further includes computing a bandlimited approximation of the phase of the variable envelope input signal from the baseband signal components of the variable envelope input signal. A phase modulated signal, based upon the computed approximation of the phase is then generated. The phase modulated signal is then applied to the signal input of the power amplifier, and an amplified output signal is generated.
Similar to the power amplifier circuit, in an alternative embodiment of the method, a bandlimited estimation of a signal""s frequency can be used, in conjunction with the generation of a frequency modulated signal for applying to the signal input of a power amplifier.
These and other features, and advantages of this invention are evident from the following description of one or more preferred embodiments of this invention, with reference to the accompanying drawings.